1. Field of the Invention
The present invention relates to image processing apparatuses, methods for controlling the same, storage media, and programs. The present invention particularly relates to a method for extracting image feature parameters used to retrieve an image.
2. Description of the Related Art
In general, various techniques of retrieving similar images have been proposed. In such a technique of retrieving similar images, image feature parameters extracted from a first image and corresponding image feature parameters extracted from a second image are compared with each other to calculate a degree of similarity, and an image retrieving processing is performed based on the calculated degree of similarity. In such similar image retrieving processing, generally, image feature parameters are extracted from images to be retrieved in advance and the images are associated with the image feature parameters and registered in an image database so that retrieving speed is improved. Furthermore, in a system in which images which are obtained by reducing original images in size (hereinafter referred to as “reduced images”) are to be displayed as retrieval results, the reduced images are registered in the image database in advance. Accordingly, high-speed display of the retrieving results is attained.
The following method is disclosed as a method for extracting image feature parameters in a system including images of different resolutions.
For example, Japanese Patent Laid-Open No. 2000-090101 discloses a method for selecting a resolution as low as possible suitable for detecting feature parameters used for image retrieving processing, high-speed extraction of usable feature parameters is attained.
Meanwhile, Japanese Patent Laid-Open No. 2001-052010 discloses a method for, when image feature parameters used for the image retrieving processing are extracted, image data of the lowest resolution is used to reduce load of extraction processing.
In a case where image feature parameters are to be extracted from first and second images of resolutions different from each other but differences between the image feature parameters to be extracted from the first image and the corresponding image feature parameters to be extracted of the second image are negligible, the image feature parameters of one of the first and second images which has the lower resolution should be extracted first so that a high-speed operation is attained. Japanese Patent Laid-Open No. 2001-052010 further discloses a technique of selecting a resolution based on capability of an apparatus so that load of the extraction processing is optimized.
However, in a case where image feature parameters which considerably vary due to different resolutions of images are to be extracted, reliabilities of the image feature parameters to be extracted should be checked. For example, a technique of extracting image feature parameters for all registered resolutions has been proposed. In this case, however, it takes considerable time to extract the image feature parameters.
Furthermore, in this case, in a case where resolution of a registered image is considerably different from a resolution of a query image, image retrieving processing is not performed with high accuracy.
Furthermore, restriction on the number of pixels required for extracting usable image feature parameters is imposed. If the number of pixels is smaller than the number of required pixels, the image feature parameters may not be extracted or unusable image feature parameters are extracted, for example.
Note that the term “usable image feature parameters” means image feature parameters which do not considerably vary due to different methods for obtaining images or a small amount of deviation of regions from which the image feature parameters are extracted. That is, when the usable image feature parameters are employed, the image retrieving processing is performed with excellent reliability irrespective of different methods for obtaining images or a small amount of deviation of regions from which the image feature parameters are extracted between the query image and the registered image.
In general, the number of pixels in an image required for extracting usable image feature parameters depends on the number of colors in the image or types of the image feature parameters to be extracted. Accordingly, the usable image feature parameters may not be extracted from an image of the lowest resolution due to some conditions of the number of colors in the image or the types of the image feature parameters to be extracted.
In particular, in a case where image feature parameters are extracted from a region of an original image, when the region to be processed is small, usable image feature parameters may not be extracted from an image of the lowest resolution corresponding to the original image.